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Zusammenfassung:
We monitor the spatio-temporal deviations of observed radiated energy, ES, for thousands of microearthquakes with respect to theoretical values, ESt, derived from a scaling model between ES and seismic moment, M0, calibrated for background seismicity in central Italy. These deviations, defined here as Energy Index (EI), provide information about thespatio-temporal evolution of mean loading stress acting on faults during the preparatory processes of the latest strong earthquakes in central Italy.
The EI parameter allow us to identify the onset of the activation phase one week before the Mw 6.1, 2009 L’Aquila earthquake. We show that foreshocks are characterized by a progressive increase in slip per unit stress, in agreement with the diffusion of highly pressurized fluids before the L’Aquila earthquake proposed by previous studies.
Furthermore, we use EI to investigate the crustal strength before and after the three largest earthquakes of the 2016 seismic sequence (i.e., the Mw 6.2, 24 August 2016 Amatrice, the Mw 6.1, 26 October 2016 Visso, and the Mw 6.5, 30 October 2016 Norcia earthquakes). We highlight the mainshocks interaction both in terms of EI and of the mean loading shear stress associated to microearthquakes occurring within the crustal volumes comprising the mainshock hypocenters. We also show the existence of a crustal volume with high EI before the Mw 6.5 Norcia earthquake, which confirms the hypothesis that the Norcia earthquake nucleated at the boundary of a large patch, highly stressed by the two previous mainshocks of the sequence.
